High-ampacity electrical joints are commonly used in energy generation and distribution systems. Currents usually range from several hundred to several thousand amperes, and the resistance of these joints varies between a few micro-ohms and several tens of micro-ohms. Owing to the high power levels involved, sporadic failure of these connections causes substantial damage to the equipment as well as reducing the overall availability of electrical energy. There are only a few preventive, non-destructive methods suitable for testing these joints. Conventional thermography, for instance, should lend itself to the detection of defective joints on the basis of observations of hot spots. However, tests generally must be performed at or close to the full, rated current expected to flow through the joint. Moreover, results are greatly influenced by factors such as the actual emissivity factor of the joint surface and its orientation with respect to the camera. Finally, electrically insulated joints may lead to erratic conclusions on the actual overheating of the joints because of ill-defined heat-transfer conditions. It is therefore not surprising that thermography inspection is not always successful in relating the recorded temperatures to the actual state of deterioration of high-ampacity joints.
The method of resistance testing of high-ampacity electrical joints is described on pages 1078 to 1082 of the September 1989 issue of "Materials Evaluation". According to the known method called the four-point method, a high-ampacity test current is run between opposite ends of a joint to be tested, and the minute potential caused by the small resistance of the joint is measured using a microvoltmeter.
The known method is effective and is considerably accurate using a high sensitivity ohmmeter. However, there is the need for effective probe assemblies for facilitating efficient contact between the conductor material of the joint to be tested and the high sensitivity ohmmeter.